This application claims priority to an application entitled xe2x80x9cMethod for Handling Idle Handoff between Base Stations Supporting Different Servicesxe2x80x9d filed in the Korean Industrial Property Office on Sep. 11, 2001 and assigned Ser. No. 2001-55852, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to an idle handoff in a mobile communication system, and in particular, to a method for setting a protocol revision value supported by a mobile station according to a protocol revision value of a target base station during an idle handoff between base stations supporting different services.
2. Description of the Related Art
In general, CDMA (Code Division Multiple Access) communication systems such as IS-95A, IS-95B, and IS-2000 communication systems, and a PCS (Personal Communications System) communication system, i.e., the typical mobile communication systems, are comprised of mobile switching centers (MSC), base stations (BS), and mobile stations (MS). The mobile switching center is connected to at least one base station, and the base station is generally divided into a base station controller (BSC) and a base transceiver system (BTS). The base station covers one cell/sector region, and controls a plurality of mobile stations included in its cell/sector region.
In the CDMA mobile communication system, call processing between a mobile station and a base station is divided into call processing by the mobile station and call processing by the base station.
The call processing by the base station includes pilot and sync channel processing, paging channel processing, access channel processing, and traffic channel processing. During the pilot channel processing, the base station transmits a pilot signal on the pilot channel. During the traffic channel processing, the base station communicates with the mobile station in a traffic state (or MS control on the traffic channel state) using forward and reverse traffic channels. During the access channel processing, the base station monitors the access channel and receives messages transmitted by the mobile station in a system access state. During the paging channel processing, the base station transmits messages on the paging channel monitored by the mobile station in an MS access state or an MS idle state.
The call processing by the mobile station includes an MS initialization state, an MS idle state, a system access state, and a traffic state (or MS control on the traffic channel state). In the initialization state, the mobile station selects and acquires a mobile communication system for communication with the base station. In the system access state, the mobile station transmits messages to the base station over the access channel, and receives messages from the base station over the paging channel assigned thereto. In the traffic state, the mobile station communicates with the base station over the forward and reverse traffic channels.
In the MS idle state, the mobile station monitors a paging channel assigned thereto. In this idle state, the mobile station can receive messages, receive an incoming call, initiate an outgoing call (or origination call), initiate registration, or initiate message transmission. Upon entering the idle state, the mobile station sets a paging channel and a data rate, and monitors the paging channel.
While monitoring the paging channel in the idle state, the mobile station may move to either another base station neighboring the current base station or another sector of the current base station. In this case, the quality of a signal received from the current base station is deteriorated, while the quality of a signal received from the neighboring base station or sector is improved. Therefore, call forward from the current base station to the neighboring base station is performed on the mobile station in the idle state. This procedure is called an xe2x80x9cidle handoffxe2x80x9d. In the mobile communication system, if it is determined that a pilot signal from a new base station other than the current base station is sufficiently higher in strength than a pilot signal from the current base station, the mobile station will initiate an idle handoff to the new (or target) base station.
Meanwhile, with the development of the mobile communication technology and the expansion of the services provided to the subscribers, the CDMA mobile communication system has evolved in the order of IS-95A, IS-95B and IS-2000. That is, the CDMA base station and mobile station have evolved from IS-95A base station and mobile station supporting the IS-95A service into IS-95B or IS-2000 base stations and mobile stations supporting the IS-95B or IS-2000 service. The existing CDMA base station and mobile station may support any one of the 3 services. As circumstances require, the IS-95A mobile station currently in communication with the IS-95A base station may move to the IS-95B or IS-2000 base station, and vice versa. Alternatively, the IS-2000 mobile station currently in communication with the IS-2000 base station may move to the IS-95A base station, and vice versa. That is, an idle handoff between the base stations supporting the different services may occur. Here, xe2x80x9csupporting the different servicesxe2x80x9d is equivalent to having different protocol revision values. For example, a protocol revision value for the IS-95A service is 2, a protocol revision value for the IS-95B service is 5 (3 to 5), and a protocol revision value for the IS-2000 service is 6.
When an idle handoff is performed between the base stations having the different protocol revision values, various problems occur.
For example, if an IS-2000 mobile station with a protocol revision value 6 initially accesses an IS-95A base station with a protocol revision value 2 upon power up, the IS-95A base station will set a P_REV field value in a sync channel message field to xe2x80x9c2xe2x80x9d before transmitting the sync channel message to the IS-2000 mobile station. The IS-2000 mobile station then compares the P_REV value included in the sync channel message with a protocol revision value supported by the IS-2000 mobile station, and operates in a mode of supporting the service corresponding to the smaller protocol revision value. That is, the IS-2000 mobile station operates in a mode of supporting the IS-95A service. At this point, if signal power received from a neighboring base station is higher than signal power received from the initially accessed base station, an idle handoff to the neighboring base station occurs. In this case, even though the neighboring base station is a base station supporting the IS-95B service or above, the IS-2000 mobile station continues to support the IS-95A service without changing the protocol revision value. Therefore, the IS-2000 mobile station will discard some of the messages transmitted by the base station supporting the IS-95B service or above. As a result, the IS-2000 mobile station cannot normally operate.
In another example, if the mobile station initially accesses an IS-95B (or IS-2000) base station upon power up, and then is idle-handed off to an IS-95A base station, the problem will become more serious. This is because although the mobile station attempts to operate in an IS-95B service mode, the target base station to which the mobile station has been handed off is an IS-95A base station, so the mobile station cannot normally operate.
FIG. 1 illustrates an operation for handling protocol revision values during an idle handoff between the base stations supporting different services according to the prior art. Referring to FIG. 1, if a mobile station receives a sync channel message in a sync state (or initialization state) upon power up, the mobile station compares a P_REV field value included in the sync channel message with a protocol revision value supported by the mobile station, and selects the smaller value as a current protocol revision value. The selected protocol revision value remains unchanged even though an idle handoff is performed later. For example, upon receiving a sync channel message from an initially accessed IS-95A base station, the mobile station selects a protocol revision value indicating the IS-95A service and maintains the selected protocol revision value. Accordingly, even though an idle handoff to the IS-95B or IS-2000 base station occurs, the protocol revision value xe2x80x9cIS-95Axe2x80x9d remains unchanged, as illustrated in FIG. 1. As a result, the mobile station cannot properly receive the services provided by the IS-95B or IS-2000 base station.
In contrast, if the mobile station has received a sync channel message from an initially accessed IS-2000 base station upon power up, a current protocol revision value is set to xe2x80x9cIS-2000xe2x80x9d. Although an idle handoff to a base station having a different protocol revision value (xe2x80x9cIS-95Axe2x80x9d or xe2x80x9cIS-95Bxe2x80x9d) is performed in the idle state, the current protocol revision value xe2x80x9cIS-2000xe2x80x9d will remain unchanged. Therefore, the mobile station having the protocol revision value xe2x80x9cIS-2000xe2x80x9d cannot properly receive the service provided by the IS-95A or IS-95B base station.
It is, therefore, an object of the present invention to provide a method for handling an idle handoff so that a mobile station can properly receive a service provided from a target base station when an idle handoff occurs between base stations supporting different services.
It is another object of the present invention to provide a method for setting a protocol revision value so that a mobile station can receive a service provided by a target base station during an idle handoff in a mobile communication system.
To achieve the above and other objects, the present invention provides a method for setting a protocol revision value for determining an environment of a service provided by a second base station by a mobile station during a handoff from a first base station to the second base station in an idle state in a mobile communication system including the first base station and the second base station neighboring the first base station, a service supported by the second base station being different from a service supported by the first base station.
In accordance with a first aspect of the present invention, the mobile station receives an extended system parameter message (ESPM) from the second base station; compares a length of the received ESPM with a length of a preset ESPM; and sets a protocol revision value of the mobile station according to a protocol revision value included in the received ESPM, if a length of the received ESPM is equal to or longer than a length of the preset ESPM.
In accordance with another aspect of the present invention, the mobile station receives an extended system parameter message (ESPM) from the second base station; compares a length of the received ESPM with a length of a preset ESPM; if a length of the received ESPM is equal to or longer than a length of the preset ESPM, compares a protocol revision value indicating a service supported by the mobile station with a preset protocol revision value and sets a protocol revision value of the mobile station according to the comparison results; compares the set protocol revision value of the mobile station with the preset protocol revision value; and if the set protocol revision value of the mobile station is equal to or larger than the preset protocol revision value, compares the protocol revision value indicating the service supported by the mobile station with a protocol revision value included in the received ESPM and sets a protocol revision value of the mobile station according to the comparison results.